An optical encoder measures a position of an object, either angular or transverse, by optically detecting marks on a scale attached to the object that moves with the object. In the simplest form, the encoder simply measures translation by counting the number of marks that move past the encoder's optical detector without ever losing count.
In a common form of such an encoder, a fixed scale and a moving scale, which have identical transparent, periodic markings on opaque backgrounds with 50% spatial duty cycle for each marking or period, are interposed between a light source and the detector. The relative locations of the transparent markings determine the amount of light which is allowed to be transmitted through each marking, e.g., full transmission, ½ or ¼ transmission, or none at all. By keeping track the number of periods which go past the fixed scale as the moving scale moves without losing count and by interpolating this variable transmission between periods, such an encoder measures relative or incremental displacement with respect to a reference or origin position. The incremental encoder does not measure absolute position which is independent of some volatile reference position. U.S. Pat. No. 5,965,879 ('879), which is incorporated by reference, teaches an absolute optical encoder with a scale having a pattern formed thereon, the pattern having a plurality of periods, each period of the plurality of periods including (a) a first portion called a fiducial bar which is identical for all of the plurality of periods and (b) a second portion which contains code bits identifying each particular period of the plurality of periods. The scale is attached to the object so that the scale moves with the object. The scale is illuminated and an image of a portion of the scale is formed on an image sensing detector. The detector means, such as a charge-coupled-device (CCD) image sensor, records an image of one period of the plurality of periods of the pattern which lies within a field of view of the detector means and outputs signals derived from the image. The field of view represents a fixed coordinate system. An analyzer receives the signals from the detector and (i) determines a location of the first portion of the one of the plurality of periods within the fixed coordinate system, (ii) decodes the second portion of the one of the plurality of periods to identify the one of the plurality of object in accordance with the location of the first portion determined in operation (i) and the identity determined in operation (ii).
A major limitation of the number of absolute optical pattern recognition encoders is a slow conversion rate, which indicates how many encoder readings the encoder system makes per unit time. That is, the conversion rate of these encoders is so low (<15 Hz) that the application space has been limited to very low bandwidths and industry has been reluctant to accept the technology for use in its applications which generally require conversion rates from 1 kHz to as high 100 kHz.
Generally, image readout from a CCD is accomplished by applying a specific sequence of electronic clocking pulses to the CCD's parallel and serial register gates. Ordinarily, full image readout from a CCD is accomplished by first applying one parallel clock pulse to shift image charge collected in the CCD's image area down by one row in the CCD's structure. When this is done, the row of image charge closest to the serial register is transferred to the serial register itself where it awaits readout in the form of an analog video signal through some sort of electronic charge-to-voltage conversion stage. (When it is desired to produce a digital image in a memory or image buffer, the analog video signal is processed through an analog-to-digital (A/D) converter.) Following the one row shift, a series of clock pulses (in number conceptually equal to the number of pixel columns in the CCD) are applied to the serial register gate to cause the image charge deposited in the serial register to shift toward the CCD's output node where a voltage suitable for A/D conversion is developed for each pixel column. The numeric brightness value for each image pixel in the image buffer is the digitally converted analog signal corresponding to the clock pulse which placed that pixel column's image charge onto the CCD's output node. This process of applying pulses to the CCD's parallel and serial register gates, repeated a number of times equal to the number of image rows in the CCD, affects a complete image readout at the CCD's highest spatial resolution.
Conversion time in previous pattern recognition encoders was limited primarily by the exposure time and the time required to read the entire image from the encoder's charge-coupled device (CCD) image sensor pixel by pixel (but not necessarily in that order). Image process time is considered to be analyzing processor platform dependent and simply devoting a faster processor to the task will always reduce image process time. However, the present invention further simplifies the image processing algorithm which improves image process time on any selected platform.
Previously, exposure time had to be long enough to produce adequately high signal-to-noise ratio in the portion of the encoder image at which bit patterns, which uniquely identity fiducial bars, are located so that the bits could be reliably detected. That the image had to be read out in its entirety, pixel by pixel, (that is, at full horizontal and vertical resolution) was necessitated by the fact that the encoder scale patterns contained substantial vertical information crucial to correct position determination through the image process, specifically in the arrangement of the row markers and code bits associated with each fiducial bar.